U.S. Pat. application Ser. No. 4,884,591 describes a glazing with variable light transmission by application of an electric potential difference successively comprising a first glass sheet, a transparent electroconductive layer, for example, with a base of indium oxide doped with tin, a layer of tungsten oxide--a transparent material which takes on a night blue coloring by insertion of protons--, an electrolytic layer consisting of a proton-conductive organic material particularly of the type of a solution of a strong acid in an organic polymer, a second electroconductive layer and finally another glass sheet. This type of glazing can be obtained starting from proven techniques of the glass transformation industry. Such techniques include depositing thin films by reactive sputtering assisted by a magnetic field and the techniques of laminating by autoclave sealing. Consequently these glazings can be prepared at costs comparable with consumer products and in large surfaces on the order of a square meter or more. The applications considered for electrochromic glazings, however, are most often limited to watch and clock displays and to precision optical techniques.
The life of such electrochromic systems, however, is very short, since microbubbles, probably due to the formation of hydrogen, appear after a few dozen operating cycles. These microbubbles are troublesome because, in a closed system such as a laminated glazing, they can be eliminated only by exchange with the ambient air.
A notable improvement to the system described above includes the insertion of a counterelectrode that can accept protons during the fading phase and release them during the coloring phases. U.S. Pat. No. 4,844,591 proposes to use a palladium layer as a counterelectrode. This layer is deposited by metallization by high-vacuum evaporation. Under these conditions the life of the system is increased by about a factor of 10. To be effective, however, the palladium layer should have a thickness greater than 10 nanometers. Under these conditions though, the light transmission of the glazing is always less than 15%, which is insufficient, for example, for a building glazing.
On the other hand, U.S. Pat. No. 4,350,414 discloses a completely solid electrochromic system consisting of stacking a series of thin layers on a transparent substrate. For example, stacking on a glass substrate: a transparent first conductive layer; a layer of electrochromic material of the tungsten oxide type; a dielectric layer of tantalum pentoxide; a layer of iridium and/or of nickel hydroxide deposited in the presence of water vapor; and a second transparent conductive layer. Insertion of a 50 to 5000 angstroms iridium hydroxide layer notably improves the variation of the optical density of the system. Optical density equals the logarithm of the ratio of the transmissions before coloring and after coloring. In other words, the iridium hydroxide layer contributes to a better contrast. However, such a system is very fragile since the last deposited layer is apparent. The system, therefore, can only be used in a unit protected by encapsulation, such as a watch or another display device. Besides, such completely solid systems are often the site of short circuits due to the presence of holes or cracks in the dielectric layer as indicated in the cited patent for tantalum pentoxide layers greater than 5 microns. Of course, the probability of a discontinuity of the layer is greater the larger the deposit surface. Thus in practice such a system can function only by choosing glass plates of very small dimensions, generally on the order of a square centimeter, and therefore is not compatible with large-surface applications.